1. Field of the Invention
The present invention relates to an abnormality detection for an electric power source used for a fixing device of the electromagnetic induction heating type.
2. Description of the Related Art
In recent years, as fixing devices of image forming apparatuses, the electromagnetic induction heating type has come into wide use.
The fixing device of the electromagnetic induction heating type includes an electromagnetic induction coil located opposite a fixing roller (belt) composed of magnetic material and electromagnetically coupled thereto, and an electric power source for causing a high-frequency current to flow through the electromagnetic induction coil to produce a high-frequency magnetic field. The high-frequency magnetic field acts on the fixing roller (belt), and eddy current flows through the fixing roller (belt), so that the fixing roller (belt) generates heat. In the fixing device thus configured, a temperature sensor for detecting a temperature of the fixing roller (belt) is provided, and the temperature of the fixing roller (belt) is controlled to a predetermined temperature by controlling the high-frequency current caused to flow through the electromagnetic induction coil based on a detection result by the temperature sensor.
If an abnormality occurs in the power source for the fixing device of the image forming apparatus, a correct high-frequency current may not flow through the coil, and the temperature of the fixing roller (belt) may fall. In this case, the sheet may be output while a toner image remains insufficiently fixed. Thus, when it is detected that the temperature of the fixing roller (belt) has fallen down to equal to or lower than a predetermined temperature which is lower than a lower limit temperature at which the fixing operation is available, an image forming operation is stopped.
However, in this method, since an abnormality can be determined only after falling below the lower limit temperature at which the fixing operation is available, there is a problem that poorly fixed sheets may be output until the abnormality is determined. In particular, with the increasing number of image formed sheets per unit time, the number of poorly fixed sheets may increase.
As measures against the above-described problem, in Japanese Patent Application Laid-Open No. 2003-295679, an image forming apparatus executes abnormality diagnosis of a power source before starting a printing operation. More specifically, the image forming apparatus once turns off the power source of the fixing device before starting the printing operation, and again turns on the power source. Then, the image forming apparatus checks detected current values Is of currents flowing through the power source before turning on and after turning on the power source, respectively. If Is>0 before turning on the power source or Is≦0 after turning on the power source, the printing operation is inhibited as it is determined that abnormality is occurring in the power source. If Is≦0 before turning on the power source and Is>0 after turning on the power source, the printing operation is started as it is determined that the power source is normal. In this way, in Japanese Patent Application Laid-Open No. 2003-295679, since abnormality diagnosis is performed before the printing operation is started, the printing operation is started after it has been confirmed that the power source is normal.
In a diagnosis method discussed in Japanese Patent Application Laid-Open No. 2003-295679, diagnosis before the printing operation is started is executable. However, since the image forming apparatus usually performs temperature control of the fixing device during the printing operation, the detected current value Is varies according to the temperature of the fixing device. For this reason, it is difficult to discriminate whether a current is not flowing in the process of the temperature control or a current is not flowing due to abnormality of the power source. If the power source is forcibly turned off during the printing operation for the purpose of diagnosis, the temperature of the fixing device may fall, and poorly fixed sheets may be output, in a case where a temperature immediately before turning off is close to the lower limit temperature at which the fixing operation is available. Moreover, in order to diagnose abnormality of the power source during temperature control, it is necessary to provide a sequence for diagnosis in a program of the temperature control. For this reason, in a diagnosis method discussed in Japanese Patent Application Laid-Open No. 2003-295679, it is difficult to determine an abnormality of the power source occurring during the progress of the printing operation.